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HORTSCIENCE 43(2):525–528. 2008. sand and grown in a greenhouse. Particle size distribution of sand was within U.S. Golf Association guidelines (USGA, 1993). Carbohydrate Greenhouse temperatures were 18 to 25 C with a photoperiod of 16 day/8 night. Natural and Efficiency of Photosystem II sunlight was supplemented with high-pres- sure sodium lamps that provided an addi- in Mown Creeping Bentgrass tional 180 mmolÁm–2Ás–1 of photosynthetic active radiation (PAR). Measurements of PAR made periodically throughout the grow- (Agrostis stolonifera L.) ing cycle exceeded 400 mmolÁm–2Ás–1. Sand Mark J. Howieson and Nick Edward Christians1 was watered daily. Department of Horticulture, Iowa State University, Ames, IA 50011 Grasses were cut at 1.3 cm three times a week with an electric reel mower (Toro, Additional index words. fructose, glucose, sucrose, double-cut, rolled, single-cut Minneapolis, MN). A wood box with holes in the top the same diameter as the pots was Abstract. Regrowth of leaf tissue after mowing is necessary to form photosynthetic leaf built to hold pots of grass in place during area required for CO2 assimilation and plant growth and development. Leaf tissue mowing. The wood box had ramps on either regrowth often is dependent on levels of reserve carbohydrates stored in leaf sheaths and side that allowed the electric reel mower to leaf bases. The objective of this study was to quantify mowing injury by measuring levels travel over pots and cut grasses. Because of fructan, sucrose, glucose, and fructose and efficiency of photosystem II (PSII) in not- the sand in pots was level with the top of cut, rolled, single-cut, and double-cut creeping bentgrass (Agrostis stolonifera L.) grown the wooden support, weight of the electric in a greenhouse. Efficiency of PSII was reduced in double-cut grasses by as much as 9% mower was evenly distributed between the compared with not-cut grasses. Fructan levels were reduced in single- and double-cut wood box and the pots. grasses compared with not-cut grasses by 52% and 45%, respectively, 36 h after mowing. Sand media were fertilized once a week Glucose levels were 31% lower in double-cut grasses compared with not-cut grasses. No with 110 mL of a complete nutrient solution differences were observed in sucrose and fructose levels among not-cut, rolled, single-cut, based on Pellet and Roberts (1963) that con- and double-cut grasses. Mowing transiently reduced fructan and glucose levels in mowed tained 102 mgÁL–1 N, 4 mgÁL–1 P, 63 mgÁL–1 K, grasses. Duration of reduction of fructan levels, magnitude of reduction in glucose levels, 17 mgÁL–1 Ca, 19 mgÁL–1 Mg, 1.2 mgÁL–1 Fe, and efficiency of PSII were greatest in double-cut grasses, suggesting that multiple 0.25 mgÁL–1 Mn, 0.10 mgÁL–1 B, 0.01 mgÁL–1 cuttings may be more damaging to plant vigor than single cutting. Zn, and 0.01 mgÁL–1 Mo. Grasses were moved to a growth chamber at least 10 months after seeding. Grasses were mowed, watered, and Mowing removes leaf tissue that grasses or used to form the disaccharide fertilized like in the greenhouse. Temperature use to acquire solar energy, thereby limit- sucrose, which is the primary sugar trans- in the growth chamber was 18 to 21 C and a ing and carbon assimilation ported in phloem (Morvan-Betrand et al., photoperiod of 16 day/8 night was maintained (White, 1973). Regrowth and initiation of 2001). Mono- and disaccharides formed from with cool white fluorescent lamps and incan- new leaf tissue after mowing is necessary to products of fructan hydrolysis are used for descent bulbs that provided 550 mmolÁm–2Ás–1 develop photosynthetic leaf area required for respiration and development of new foliage of PAR. Grasses were acclimated in the production of carbohydrates (Parsons et al., after defoliation (Amiard et al., 2003; Morvan- growth chamber for at least 3 weeks before 1983). Rate of leaf tissue regrowth often Betrand et al., 1999). initiation of treatments. Although no formal depends on levels of carbohydrates present Golf course greens often are double-cut measurements were recorded, growth rate in grass tissue before defoliation, suggesting or rolled to improve uniformity and increase and clipping yield of grasses were similar that plants rely on carbohydrate reserves to ball roll (Nikolai, 2004). Double-cutting is when grown in the greenhouse and growth provide energy and raw materials to rede- clipping grasses twice in the same mowing, chamber. velop leaf and shoot tissue (Davidson and whereas grasses are rolled by using light- Grasses were not cut for 3 d before Mithorpe, 1966; Donaghy and Fulkerson, weight drums or cylinders to lightly compact application of mowing treatments. Mowing 1998; Morvan-Betrand et al., 1999). the grass surface (Nikolai, 2004). Grasses treatments included rolling, single-cutting, The primary reserve carbohydrate of also are double-cut in the overlap areas be- and double-cutting. Grasses were rolled creeping bentgrass (Agrostis stolonifera L.) tween reels in gang mowers or between with a 108-kg electric greens mower without is fructan. Fructan is a polymer of fructose subsequent passes of walk-behind mowers. the reel engaged, whereas single-cut grasses with a terminal glucose moiety (Chatterton These double-cut areas often develop ‘‘over- were mowed once and double-cut grasses et al., 1989). Increased catabolism and de- lap marking’’—a distinct U-shaped groove in were mowed twice at 1.3 cm. Harvests of creases in levels of fructans have been which grasses may become discolored and plant tissue for carbohydrate analysis and observed in grasses in response to defoliation exhibit reduced growth rates. It is not known measurements of efficiency of PSII were (Morvan-Betrand et al., 2001; Prud’homme if multiple cuttings limit the recuperative made 0, 12, 24, 36, 48, and 60 h after treat- et al., 1992; Volenec, 1986). The magnitude potential of mowed grasses more than sin- ment. Creeping bentgrass stubble was har- of reduction in levels of reserve carbohy- gle-cutting or rolling. The objective of this vested frelected pots per treatment with a drates often is dependent on the severity study was to quantify mowing injury by razor blade at each sampling time. The tissue of defoliation. In general, lower mowing measuring levels of fructan, sucrose, glucose, was immediately frozen under liquid nitro- heights and more frequent cuttings cause fructose, and efficiency of photosystem II gen and stored at –80 C for analysis of greater reductions in carbohydrate reserves (PSII) in not-cut, rolled, single-cut, and carbohydrates. (Donaghy and Fulkerson, 1998). Fructose double-cut grasses. A portable fluorometer (PAM-101; Walz, hydrolyzed from fructan can be converted Effeltrich, Germany) fitted with a 2-mm to the monosaccharide glucose by isomerase Materials and Methods fiberoptic cable was used to measure light- adapted efficiency of PSII (Fm# – F#/Fm#)of Two identical, independent experiments individual leaf blades, where Fm# is fluores- # Received for publication 11 Dec. 2006. Accepted were repeated in time. Creeping bentgrass cence in the light-adapted state and F is for publication 20 Nov. 2007. (Agrostis stolonifera L.) cv. L-93 seeds were initial fluorescence (Genty et al., 1989). 1To whom reprint requests should be addressed; planted at a rate of 24 kgÁha–1 in 12.7-cm Efficiency of PSII was measured at the e-mail [email protected] diameter by 11.4-cm deep pots of calcareous adaxial tip of 10 individual light-adapted

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leaves selected at random from pots before tissue sampling. A 0.5- to 1.0-g sample of frozen leaf tissue was crushed under liquid N2 and suspended in 10 mL of high-performance liquid chro- matography (HPLC)-grade water to extract soluble carbohydrates, whereas a subsample was dried in a 68 C oven for 72 h for dry weight analysis. The mixture was heated to 95 C for 30 min and then centrifuged at 1000 g for 10 min (Livingston, 1991). Samples were deionized by passing a 100-mL aliquot of supernatant through a 1-mL chro- matography column packed from top to bottom with 120 mL cation exchange resin [Dowex (Sigma Aldrich, St. Louis) 50 ·8- 400, H+-form], 100 mL insoluble polyvinyl- pyrrolidone, and 120 mL anion exchange resin [Amberlite (Sigma Aldrich, St. Louis) CG-400 II, formate form]. Columns were eluted twice with 250 mL water. Elutants were combined and filtered through a 0.45- mm nylon filter for HPLC analysis (Amiard et al., 2003; Bachmann et al., 1994). Water-soluble carbohydrates were ana- lyzed by using HPLC. Sucrose, glucose, fruc- tose, and fructans were separated on a Sugar- Pak column (300 · 6.5 mm; Waters, Milford, MA) and were isocratically eluted at 85 C by using 0.1 mM disodium calcium salt of EDTA at a flow rate of 0.5 mLÁmin–1.Sepa- rated carbohydrates were identified by exter- nal standards and quantified by using a model 2410 differential refractometer (Waters). In both experiments, treatments were completely randomized and three inde- pendent replicates were used to determine concentrations of carbohydrates at each sampling time. The general linear model of SAS (SAS Institute, Cary, NC) was used for analysis of variance of experiment, treat- ment, time, and their interactions. Values reported in the text and figures are represen- tative of the highest-order interaction or main effects significant at the 0.05 level. Treatment means were separated by using Fisher’s least significant difference. Fig. 1. Efficiency of photosystem II of not-cut, rolled, single-cut, and double-cut ‘L-93’ creeping bentgrass over time in Expt. 1 (A) and Expt. 2 (B). Means at each sampling time labeled with the same letter Results and Discussion are not different at the 0.05 level by using Fisher’s least significant difference to separate means. Each data point is the mean of 30 observations. Efficiency of PSII of single-cut and rolled grasses was reduced by 5% compared with not-cut grasses 12 h after treatment in exper- iment one (Fig. 1A). Double-cut grasses 1999), which can limit the efficiency of action between experiment and treatment had a reduction in efficiency of PSII of PSII. Decreases in efficiency of PSII often existed. Fructan levels were 33% lower in 9% compared with not-cut grasses 12 h after are thought to be caused by an applied stress double-cut grasses compared with not-cut treatment in experiment one (Fig. 1A). that has damaged the photosynthetic ap- grasses 24 h after treatment. Single-cut grasses Twenty-four hours after treatment, efficiency paratus. This results in reduced efficiency had 22% less fructan than not-cut grasses of PSII of single- and double-cut grasses of light transfer in photosynthetic electron 24 h after treatment (Fig. 2). Thirty-six hours was reduced by 4% in Expt. 1 (Fig. 1A). In transport (Maxwell and Johnson, 2000). after treatment, fructan levels of single- and Expt. 2, single- and double-cut grasses had Although reductions of efficiency of PSII double-cut grasses were 52% and 45%, re- reductions in efficiency of PSII of 2% and in single- and double-cut grasses were likely spectively, lower than not-cut grasses. Fruc- 2.5%, respectively, compared with not-cut too small and transient to be considered tan levels were 38% lower in double-cut grasses 12 h after treatment (Fig. 1B). Effi- sustained of PSII, they none- grasses compared with not-cut grasses 48 h ciency of PSII of single- and double-cut theless indicate a wound response. In this after treatment (Fig. 2). grasses was reduced by 3% compared with instance, the magnitude of reduction in effi- Reduced levels of fructans also have not-cut grasses 36 h after treatment in Expt. 2 ciency of PSII indicates that double-cutting is been observed in mown perennial rye- (Fig. 1B). more injurious than single-cutting. grass (Lolium perenne L.) (Donaghy and A common response of many plants to Values reported for levels of fructan are Fulkerson, 1998; Sullivan and Sprague, wounding is to increase formation of reactive means averaged over both experiments. Fruc- 1943) and tall fescue (Festuca arundinacea oxygen species (Orozco-Ca´rdenas and Ryan, tan level data were pooled because no inter- Screb.) (Volenec, 1986). In these experiments,

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the magnitude and duration of reduction in whereas glucose levels were reduced in tained levels of fructose in single- and dou- the level of fructan were greatest in double- single- and double-cut grasses compared with ble-cut grasses (Prud’homme et al., 1992). cut grasses indicating that multiple cut- rolled and not-cut grasses (Fig. 3). Because Likewise, sucrose likely also was resynthe- tings likely cause greater stress than single only the main treatment effect was signifi- sized from free fructose for transport to cuttings. cant, levels of glucose were pooled over elongating leaves to provide energy for Changes in levels of fructose, glucose, experiment and time. Levels of glucose were formation of new leaf tissue (Pollack and and sucrose in response to mowing treat- 31% lower in double-cut grasses compared Cairns, 1991). Amiard et al. (2003) and de ments were variable. No differences in levels with not-cut grasses (Fig. 3). Visser (1997) have demonstrated that prod- of sucrose or fructose were observed among It is likely that hexose sugars liberated ucts of fructan hydrolysis are used for respi- treatments in either study (data not shown), by hydrolysis of fructan after mowing main- ration and development of new leaf tissue until foliage has regrown and rate of CO2 assimilation is sufficient to support plant growth and metabolism. However, steady- state labeling experiments and assays of car- bohydrate metabolism rates would be needed to confirm this hypothesis. Mowing reduced efficiency of PSII and resulted in transient decreases in the levels of fructan and glucose in creeping bentgrass. The duration and magnitude of reduction in efficiency of PSII and fructan levels were greatest in double-cut grasses, indicating that multiple cuttings may be more damaging to plant growth and development. Mowed grasses, regardless of the number of cuttings, were able to regrow sufficient leaf tissue to support plant metabolism and restore levels of reserve carbohydrates. Narra et al. (2004) often observed similar carbohydrate levels in creeping bentgrass cut at different heights in clippings collected 4 to 6 d after mowing. However, additional stresses like high tem- perature (Xu and Huang, 2000) that also reduce levels of reserve carbohydrates may limit recuperative potential of mown creep- ing bentgrass, especially when grasses are double-cut. Additional field trials should be Fig. 2. Fructan concentration of not-cut, rolled, single-cut, and double-cut ‘L-93’ creeping bentgrass. Data performed that monitor levels of carbohy- averaged over experiments. Means at each sampling time labeled with the same letter are not different drates and turfgrass quality over time to at the 0.05 level by using Fisher’s least significant difference to separate means. Each data point is the support this hypothesis. mean of six observations.

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